The inhibitive behaviour of Vitamin C (ascorbic acid), as a type of green inhibitor, on the corrosive
behaviour of stainless-steel (SS) X4Cr13 within an aqueous solution of hydrochloric acid (HCl) was
studied within a concentration range of 0.01 – 5.0 M HCl. Potentiodynamic polarization and
electrochemical impedance spectroscopy (EIS), was used for the measurements at 25 °C. The
polarization curves revealed that the studied mixtures of the chosen inhibitor represented mixed-type
of the inhibitor. A good inhibitive efficiency for Vitamin C at chosen concentration of 10
-2
and 10
-3

Acidic solutions are extensively used within industry for industrial-equipment cleaning, oil-
well acidization, and the pretreatments of compositions [1–3]. Hydrochloric acid is widely- used in
pickling solutions. The addition of an inhibitor is necessary to prevent corrosion i.e. to reduce the
dissolution of metals within these acidic environments. The usage of chemical corrosion-inhibitors is
common during production and processing operations. The addition of high molecular weight organic
compounds, such as surface active agents, to acid solutions has been proved to be an effective method
to reduce the rate of corrosion of metals, as shown recently. Moreover, ionic and non-ionic surfactants
have been reported to be corrosion inhibitors for metals like copper, aluminium and mild steel [4-14].
The development of new chemicals is particularly challenging, as they would need to maintain good
protection of the materials under a variety of conditions, whilst being environmentally acceptable.
There is increasing concern about the toxicity of corrosion-inhibitors throughout in industry. Any toxic
effects not only affect living organisms but also poison the earth. Therefore, over recent years, the
Int. J. Electrochem. Sci., Vol. 8, 2013

1512
traditional approach regarding corrosion inhibitors has gradually changed, owing to the increasing
interest and attention of the world towards environmental problems, towards protecting the
environment, the hazardous effects of the using of chemicals on the ecological balance. It is believed
that toxic inhibitors as widely-used within industrial processes should be replaced by new
environmentally friendly ones.
As certain types of vitamins have similar structures to those of conventional organic inhibitors,
research efforts have therefore begun to focus on non-toxic, or green corrosion. Heteroatoms, such as
P, S, N, and O, which can donate lone pairs of electrons, can be found in Vitamins such as Vitamin B
(thiamine), C (ascorbic acid), E (tocopherol) etc.[15–19].
The purpose of this work was to study the inhibitive action of Vitamin C (ascorbic acid), as a
type of green inhibitor, on the corrosive behaviour of stainless-steel (SS) X4Cr13 in an aqueous
solution of hydrochloric acid (HCl) within a concentration range of 0.01 – 5.0 M HCl (M = mol L
-1
).
The most desirable surfactant inhibitor is the one that ensures high inhibitive efficiency at low
surfactant concentrations. The chosen concentrations of added Vitamin C were 0.01 and 0.001 mol L
-1
.
Therefore, the goal of this work was to find the highest concentration of HCl, where, the
chosen concentration of added Vitamin C (0.01 or 0.001 M of Vitamin C), would be enough to achieve
a satisfactory degree the inhibitive efficiency.
Two different electrochemical methods were used for this purpose, i.e. classical
potentiodynamic measurement and electrochemical impedance spectroscopy.

2. EXPERIMENTAL PART
The conventional three-electrode configuration was applied in order to conduct the
potentiodynamic studies. All the potentials were measured against the saturated calomel electrode
(SCE), and the counter electrode was made from Pt. The potentiodynamic current-potential curves
were recorded by automatically changing the electrode potential from -0.7 to not more than -0.1V with
a scanning-rate of 5 mVs
÷1
. EIS measurements were carried out within the 60 kHz –1 mHz frequency
range at a steady open circuit potential (OCP) disturbed by an amplitude of 10 mV. Nyquist and
polarization plots were obtained from the results of these experiments 120 mins after the working
electrode had been immersed in the solution, in order to allow stabilization of the stationary potential
All the experiments were carried at 25°C ± 1°C. The measurements were performed using the
Solartron 1287 Electrochemical interface and the Solartron 1250 Frequency response analyser. Data
were collected and analysed using CorrView, CorrWare, Zplot, and ZView software, as developed by
Scribner Associates Inc. The surface coverageu, was calculated via the kinetic parameters measured
during corrosion processes, as well as the polarisation resistance R
p
, and the corrosion current density
i
corr
.

Vitamin – C

Int. J. Electrochem. Sci., Vol. 8, 2013

1513
3. RESULTS AND DISCUSSION

Electrochemical results
The effects of the presence of Vitamin C on the current–potential characteristics displayed by
the polarisation curves of ferritic stainless steel type X4Cr13 in HCl is presented in Figs. 1 and 2.
It is clear that the addition of Vitamin C hindered any acidic attack on the steel electrode. A
comparison of curves showed that, with respect to the blank, a noticeable decrease in the anodic
current-peak was observed at concentration of HCl from 0.01 to 0.1 mol L
-1
. Furthermore, an
appreciable lowering of the cathodic current may be observed from these figures. This result suggests
that the addition of the selected vitamin reduced anodic dissolution, and retarded the hydrogen
evolution reaction (a mixed-type of inhibitor). No anodic current peak was observed at concentrations
of HCl higher than 0.1 mol L
-1
.
It is necessary to mention that at concentration of HCl c = 1.0 M with and without addition of
the Vitamin C, the profiles on voltammogram (Fig. 1) obtain flat profiles. The flat portion extends to
about 100 mV in both directions, with regard to the corrosion potential. The best inhibitive efficiency
is expected within the potential region where the current is at minimum. The flat portion was
additionally extended by about 50 mV in both directions with the addition of 0.01 M Vitamin C in the
1.0 M HCl solution. At the concentration of HCl higher than 1.0 mol L
-1
, this effect becomes
negligible.
The electrochemical parameters obtained from these polarization curves, corrosion potential
(E
corr
), corrosion current density (i
corr
), the polarisation resistance (R
p
) and the inhibition efficiency, are
shown in Table 1. The polarisation resistance was obtained from linear polarisation within the potential
range of ±10 mV with respect to E
corr
. Extrapolation of the Tafel line allowed us to calculate the
corrosion current density i
corr
. All the parameters were determined simultaneously by CorrView
software. In the presence of Vitamin C, lower values of the i
corr
we recorded. Actually, the inhibitive
efficiency increased with the concentration of Vitamin C, but at concentrations of HCl higher than 0.1
mol L
-1
, these values became negligible.

where the notations i
corr
and R
p
were used for those measurements without added-surfactant,
whilst the primed quantities i
corr
' and R
p
' were applied when the Vitamin C was added to an HCl
solution. The adsorption of organic molecules may be explained by the presence of an oxygen atom (a
hetroatom), t electron of aromatic rings and electron donating groups. Hetro-atoms, such as oxygen,
are the major adsorption centres within organic compounds because of their interactions with metal’s
surface [20]. Adsorption can also occur via electrostatic interaction between a negatively-charged
surface, as provided by a specifically adsorbed anion (Cl
-
) on the metal’s surface (iron, steel), and the
positive charge of the inhibitor [21].
Impedance measurements of the SS electrode type X4Cr13 at its open circuit potential after 2 h
of immersion in HCL solution alone, and in the presence of various inhibitors’ concentrations, were
performed over a frequency range from 60 kHz –1 mHz.
-350.0
-300.0
-250.0
-200.0
-150.0
-100.0
-50.0
0.0
50.0
-150.0 0.0 150.0 300.0 450.0 600.0 750.0
Z
i
m
/

Figure 3. EIS Nyquist plots for stainless steel type X4Cr13 in HCl with and without with and without
added x M of Vitamin C at 25ºC.

Figure 4. The electrochemical equivalent circuit used for simulating the impedance data of SS type
X4Cr13 in HCl with and without with and without added x M of Vitamin C at 25ºC.

In order to compare the corrosion-behaviours of different solutions, Figs. 3 show the Nyquist
diagrams of SS type X4Cr13. The charge transfer resistance (R
ct
) and the interfacial double layer
capacitance (CPE) values were derived by using the equivalent circuit, as shown in Fig. 4.
The results obtained by EIS (Table.2) in some way, confirm the interpretation mentioned above
(polarization measurements). In the blank solution the C
dl
-values have decreased with increasing
concentration of HCl. On the basis of this phenomenon it could be assumed that, Cl
–
anions are first
adsorbed on the metal surface, they charge it negatively. In highly acidic solution, Vitamin-C exists in
the protonated form. So, molecules of Vitamin-C may adsorb through electrostatic interactions
between positively-charged molecules and negatively-charged metal surfaces. But, with addition of
Vitamin C the C
dl
-values were start to decrease in the opposite direction. This effect probably indicates
on the porous nature of the adsorbed film at the concentration higher than c = 0.1 M of HCl.
A good inhibitive efficiency for Vitamin C at chosen concentration of 10
-2
and 10
-3
mol L
-1
on
the SS type X4Cr13 in HCl was limited to concentrations of HCl below 1.0 mol L
-1
.

4. CONCLUSIONS
Results show that Vitamin-C exhibits a dual character. In some cases it acts as corrosion
inhibitor and in the other as an activator of corrosion processes.
A comparison of curves showed that, with respect to the blank, a noticeable decrease in the
anodic current-peak was observed at concentration of HCl c = 0.01 mol L
-1
. No anodic current peak
was observed at the concentration of 1.0 M of HCl. In that case the flat portion extends to about 50 mV
in both directions with regard to the corrosion potential. In continuation just the increasing of the
current density was obtained.
A good inhibitive efficiency for Vitamin C at chosen concentration of 10
-2
and 10
-3
mol L
-1
on
the SS type X4Cr13 in HCl was limited to concentrations of HCl below 0.1mol L
-1
.